Eye surgery is a complicated and delicate process. One common eye surgery is cataract extraction. There are currently several methods of acceptable cataract extraction, including phacoemulsification. Phacoemulsification is not in itself new, but as currently done has many problems.
Phacoemulsification involves the generation of an ultrasonic signal which is a series of cyclical mechanical vibrations in a frequency range beyond that detectable by normal human hearing. The ultrasonic signal is generated by a transducer that is driven by an electrical signal in a frequency range between 20 and 100 kilohertz in equipment presently available for this application. Typically the transducer mechanism includes either piezoelectric or magnetostrictive elements.
The energy resulting from the ultrasonic signal is coupled to the human lens by a needle attached to the transducer. Typically the needle is made from an inert alloy of titanium or stainless steel. Once coupled to the human lens, the ultrasonic energy fragments and emulsified the cataract. Once this nuclear material is fragmented, however, it must be removed from the eye. In order to do this, the ultrasonic needle is hollow, and an aspiration system is connected to the hollow area in order to remove the fragmented particles. A balanced salt solution is also injected in order to maintain the stability or pressure, and this infusion occurs around the vibrating titanium needle through a sleeve.
An example of such a phacoemulsification unit is shown in U.S. Pat. No. 4,223,676, the disclosure of which is hereby incorporated by reference. Current phacoemulsification surgery allows the surgeon to choose either a fixed phaco mode in which the power setting to the transducer is fixed, or a linear mode in which the phaco power can be changed by the power peddle. In the fixed mode, the phaco unit is either on or off depending on whether the peddle is depressed or not. The value of power setting is preset. In the linear mode, the further depression of the peddle varies the amount of power to the transducer and thereby the ultrasonic energy. The aspiration during this operation is preset. A third mode of phacoemulsification which has been recently introduced keeps the phaco power fixed and varies the aspiration depending on the foot peddle.
The inventor of the present invention has recognized a problem which exists in these prior operations. In order to fully understand this, one must consider the structure of the lens of the human eye. FIG. 1 shows diagrammatically a human lens which has an outer, fine, transparent tissue or capsule shown as layer 100. Anterior to this is a soft material known as the cortex 102, which surrounds the transition layers 104. The nucleus of the lens is a hard, compressed lens material shown as 106. The inventor of the present invention has first noted that in these soft outer cortical layers, little aspiration is required, but more aspiration is required in the harder transitional layers and even more in the hardest nucleus layer. However, posterior to the hardest nucleus layer is a less hard transitional layer followed by a soft cortex. A majority of the complications during eye surgery are caused not by the amount of phacoemulsification, but by overaspiration in conjunction with the emulsification causing a "punch through" through the posterior lens capsule. This is particularly dangerous since the center of the lens needs more energy (aspiriation and emulsification) than the outer soft cortical layer, and therefore there is more possibility of punch-through at this higher energy level and high aspiration level.
Eye surgery involves forming an opening in the front of the capsule, and locating the phaco needle first into the soft cortex. At this time the needle will experience a minimal load in the soft cortex. As the needle goes further into the nucleus which is progressively harder, the mechanical load increases. After passing through the nucleus, the process reverses, and the mechanical load will quickly decrease. It is at this point that the inventor of the present invention has found that the control of aspiration becomes critical. Over-aspiration at this time can cause the posterior capsule to be ruptured. However, determination of the relative hardness of these layers has previously been left to the observation skills and manual skills of the surgeon. However, the surgeon has many other things on his mind and also simply may not be able to react fast enough in order to properly change the aspiration amount.
The inventor of the present invention has recognized that a hard nucleus consumes more energy than a soft nucleus, thereby changing the impedance, and more specifically the mechanical impedance, introduced to the ultrasonic tip. According to the present invention, this difference is fed back to a microprocessor in order to modify the aspiration system dependent on the hardness of the material being operated upon. This reduces the problem of "punch through" because it allows automatic checking of the hardness of the material and automatic adjustment of the aspiration delivery in a way which is faster than could ever be done using human reflexes. Such a system has never been described in the prior art. One way in which this is done is by detecting mechanical impedance of the tissue, using, for example, a sensor to detect response to a stimulus.
One general feature of the present invention is the recognition by the inventor of the present invention that soft tissue requires a low stroke or low velocity and that hard tissue requires a high stroke and high velocity. The mechanical impedance of any material including the human eye is a function of the density .rho. and sound velocity C. It usually has a resistive component R.sub.c and a reactive component X.sub.L. Compliant or deformable tissue presents primarily a resistive impedance to the driving force. Non-compliant or non-deformable tissues are primarily a reactive impedance. In other words, soft tissue will be more resistive and hard tissue will be more reactive.
One approach to detecting mechanical impedance from a piezoelectric hand piece is to read the driving voltage and current. Here not only magnitude but also phase will be monitored where zero phase difference will indicate a resistive load on soft tissue. A large phase difference would indicate a reactive load or hard tissue. Another approach would include determining the resonant frequency of the loaded hand piece in relation to a reference, which can be the resonant frequency of the unloaded hand piece. If the transducer is formed as a half wavelength straight bar, its resonant frequency will not change for purely resistive loads and can be determined according to the equation ##EQU1## where f is the operational frequency, c is the speed of sound in the bar, and x is the length of the bar. For a purely reactive load, the resonant frequency is determined by the equation ##EQU2## where XL is the reactive load and Z0 is the characteristic impedance of the bar. If the transducer is made as a step horn type to provide amplification of the displacement, the resonant frequency will change for either resistive or reactive loads. A typical step horn device is shown in FIG. 14 with its two parts 1050 and 1052. The lengths X of parts 1050 and 1052 are equal to one another but their areas differ by a factor of N&gt;10.
For a device of this type, the resonant frequency is determined according to the equation ##EQU3## where X is the length shown in FIG. 14 and Z0 is the characteristic impedance of the transducer material as shown in FIG. 15. The part 1052 has the impedance Z0 while the part 1050 has the characteristic impedance N.times.Z0. For purely reactive loads the resonant frequency can be determined from the equation ##EQU4##
These equations are general and exemplary and different needle/transducer arrangements could use different equations.
Many attempts have been made in the prior art in order to attempt to automate operation processes. U.S. Pat. No. 4,223,676 is one such attempt and defines one type of ultrasonic aspirator of the type previously described above. Column 8 of this patent recognizes that frequency fluctuates during the course of an operation, and in order to attempt to maintain the amount of power delivery as constant, this patent teaches monitoring actual and expected parameters of the system. The difference between these two parameters is fed back in a feedback loop to control the stroke level of the vibrator. Therefore, while the power of the system is controlled, there is no teaching of controlling the amount of aspiration, and as such the problem of "punch through" would still remain in this system.
Similarly, U.S. Pat. No. 3,964,487 teaches a structure which monitors the impedance of the electric cutting apparatus, and feeds back this impedance to determine the amount of power to be provided. This device does not teach controlling the amount of aspiration, and therefore would not alleviate the problem of "punch through".
Similarly, U.S. Pat. No. 4,126,137 teaches sensing of the impedance of the tissues to set the amount of drive to an electro-surgical unit.
U.S. Pat. No. 4,024,866 relates to a device which teaches controlling the amount of suction in a suction conduit for eye surgery. Column 7, lines 24 teach that an upper limit is placed on the amount of suction to prevent an excessive amount of suction. While this might provide an upper limit, it does not help the user to obtain better control and better feedback within the system